Aerial drone

Why do light aircraft with piston engines make so much noise, even when they are far away? They can be very intrusive, especially in secluded landscapes such as Dartmoor National Park. Are they fitted with silencers like road vehicles are? And if not, why not?

• Designers traditionally gave little thought to noise in small aircraft, concentrating instead on lightness, simplicity, payload and performance. Research has shown that the engines and propellers make most of the noise.

Silencing them for military reconnaissance proved effective. During the Vietnam war, for example, the Lockheed YO-3 was practically inaudible at an altitude of 300 metres. But muffling the engine, especially the air intake and exhaust, is costly in terms of weight and affects performance: the efficiency of internal combustion engines is sensitive to how easily they can be fed air and how well they “breathe”.

“In the Vietnam war, the Lockheed YO-3 was practically inaudible at an altitude of 300 metres“

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However, silencing engines proves less demanding than silencing propellers, for which blade width, length, shape, rotation rate, choice of material (wood, metal or synthetics) and number of blades are all factors to consider. Each of these also affects cost, weight, performance and safety. In the competitive business of small-aircraft manufacture, noise abatement and sophistication are understandably lower priorities than cost and proven design.

But as the number of airfields and light aircraft in population centres increases, the demand for sound abatement is intensifying. As a result, muffled engines and cheap, efficient scimitar-shaped blades, which are quieter than other types, can be expected in new standard designs.

Jon Richfield, Somerset West, South Africa

• I can suggest a number of reasons why light aircraft make so much noise. The first is that they are predominantly flown as a hobby, so cost is a major factor. Therefore light aircraft need to be just that: light. The heavier a small plane is, the bigger the wings and engine need to be, all other things being equal, and the more expensive to run.

Adding silencers incurs undesirable cost, as well as complexity and weight. For that reason, most light aircraft don’t have them.

Silencers also reduce engine power and, because light aircraft tend to be built with the smallest and least powerful engines possible, anything that reduces power is highly unwelcome.

Finally, small planes tend to fly slowly, relatively close to the ground and below the clouds. This means the noise is louder and more persistent at ground level than it would be if the plane were flying higher and above a muffling cloud layer.

Manek Dubash, Lewes, East Sussex, UK

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